I. Field of the Invention
The present invention relates to ISDN communications systems and, more particularly, to a power detection circuit located at a remote station which consumes little power while providing good isolation between the remote station logic and the transmission lines.
II. Prior Art
A major step in the evolution of common carrier facilities from an analog environment to a completely digital environment lies in the implementation of an integrated services digital network (ISDN) wherein a digital channel is provided directly to the end user. In such a network, the equipment on the user's premises, such as a digital telephone or data terminal, generates a digital signal for being transmitted directly to the common carrier's switching office over local loops that are engineered to carry digital signals. The interface network is documented in the form of standards composed by the International Telegraph and Telephone Consultative Committee (CCITT) for international standards and by the American National Standards Institute (ANSI) for U.S. standards.
Similar to a standard analog telephone system, ISDN provides power to remote stations (on the user's premises) for powering the user's equipment. As can be seen in the simplified block diagram in FIG. 1, an ISDN network 10 utilizes two transmission line pairs to each remote station: a transmit pair 16 for conveying data to the remote station 12, such as a data terminal equipment (DTE), from a central office 14 and a receive pair 18 for conveying data from the remote station 12 to the central office 14. Because both the transmit pair 16 and the receive pair 18 are terminated by transformers 20, DC power is provided by a power source 24 in the central office 14 to the remote station 12 via the center terminal 22 of each transformer 20. In this way, the differential current (and voltage) across each pair is zero.
In order to implement the full potential of ISDN, the CCITT and ANSI standards define an elaborate state machine. This state machine integrates the physical world (i.e., wires, voltages, currents) into an abstract world of virtual communication links and networks. To fully pass through all the required and optional states, the hardware must be aware that it is currently connected to a functioning ISDN line, and must be capable of detecting when a connection has been physically interrupted. The required method of doing this is to use a power sensor 26 to determine if power is being supplied over the transmission lines. This method is particularly practical when the user's equipment does not use the power provided over the ISDN lines but rather consumes the user's own power. A problem arises, however, in that the user is severely limited by the international and U.S. standards as to the amount of power which can be consumed by the power sensor 26 in assessing the availability of a functioning ISDN network. Typically, the power sensor circuit 26 cannot consume more than 3 milliwatts (mW), the power consumption limit varying slightly depending upon which standard is referenced.
In addition, because of the threat of lightning striking the ISDN lines or the possibility of ISDN and high power lines crossing, the power sensor circuit 26 must provide a high degree of voltage isolation between the ISDN lines and the remote station logic. Requirements for isolation range from 1500 to 3500 Volts (V), again depending upon which standard is referenced.
In the past, transformers have been used as a means of providing a high degree of isolation for data signals, but are not suitable for DC applications.
Electrical devices such as opto-couplers have been used to meet the required dielectric isolation requirements but typically consume more than 3 mw. To operate effectively and consistently, opto-couplers require at least 1 milliams (mA) of drive current, preferably 10 mA, if detection over temperature extremes and over the life of the opto-coupler is desired. As voltage of the central office power supply 24 ranges from 24V to 56.5V, the maximum current that is allowed to flow through the detection circuit without exceeding the power consumption limit ranges from 22.6 to 56.5 micro-amperes, respectively. As a result, the use of opto-couplers in such a remote station application requires much more than the 3 milliwatts of power consumption that ISDN standards allow.
Therefore, it is desirable to have a method for detecting power at an ISDN remote station that consumes little power (&lt;3mW) and provides a high degree of isolation (&gt;3000V).